Hydraulically-operated control system for a screw compressor

ABSTRACT

A power system includes a power unit, a screw compressor, a hydraulic pump, and a hydraulic control system. The screw compressor is connected to the power unit to be driven thereby for compressing air. The screw compressor has an air intake opening and an adjustable air intake valve mechanism for opening and closing the air intake opening. The hydraulic pump pressurizes hydraulic fluid, and the hydraulic control system is operably connected to the pump and to the air intake valve mechanism for utilizing the pressurized hydraulic fluid to adjust the air intake valve mechanism. The compressed air is delivered to an air reservoir which returns lubrication oil back to the compressor in quantities commensurate with the degree of openness of the air intake opening.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a control system for controlling operation of a screw-type screw compressor with hydraulic pumps, especially earth working equipment such as mining machines and drilling rigs.

[0002] A conventional mobile drilling rig 10, depicted in FIGS. 1 and 2, comprises a platform 12 below which a drivable ground support is attached, such as a pair of rotatable wheels (not shown) or two rotatable endless carrier tracks 14, disposed on respective sides of the platform. An operator's cab 16 is disposed at a rear end of the platform. Situated on the platform in front of the cab 16 is a drilling assembly 17 for drilling holes downwardly into the ground. The drilling assembly 17 includes a swingable beam 18, such as a mast whose lower end is pivotably connected to a fixed pedestal 18 a to be swingable about a horizontal axis extending perpendicularly to a front-to-rear extending longitudinal axis A of the rig. Thus, the mast 18 can be swung by hydraulic devices 21 from the vertical working state depicted in FIG. 1 to a horizontal transport state (not shown) in which the free end of the mast sits on a mast rest 20 disposed at a front end of the platform.

[0003] Mounted on the mast is a hydraulically powered raising/lowering mechanism for raising or lowering a drill string 25 having a drill bit at its lower end. The drill string is comprised of series of interconnected drill rods that are stored in a carousel mounted on the mast. Hydraulic devices such as motors or cylinders are provided for rotating the carousel.

[0004] At least one, but usually several, hydraulic pumps 27 are provided for supplying pressurized fluid to the various hydraulic devices and hydraulic motors.

[0005] In order to flush cuttings from a hole as it is being drilled, it is common to direct compressed air downwardly through the drill string to the front face of the drill bit. The cuttings become entrained in the airflow and are brought to the surface as the air travels upwardly along the annulus formed between the wall of the hole and the exterior of the drill string. The compressed air also serves to cool the cutting elements of the drill bit, and/or to drive an impact piston in percussion-type tools. The compressed air is produced by a compressor, typically a screw compressor 22 which, as shown in FIG. 2, comprises a pair of intermeshing screws 23 a, 23 b rotating about parallel axes. The compressor delivers compressed air to an air reservoir 19 from which it is conducted to the drill string. Lubricating oil is mixed with the compressed air for lubricating the compressor. The lubricating oil is separated from the compressed air within the reservoir 19 and is conducted back to the compressor and the compressor gear box.

[0006] In order to drive the screw compressor 22 and the hydraulic pumps 27, it is conventional to employ power unit 26, such as a fuel-driven engine (e.g., a diesel engine) or an electric motor, for example, the power unit being cooled by a fan 29 that is driven by pressurized hydraulic fluid from one of the pumps.

[0007] The compressed air needs of such a drilling machine occur only during a drilling sequence, i.e., for supplying flushing air for flushing cuttings and/or driving the impact piston of a percussive tool. Therefore, it will be appreciated that for long periods during operation of the drilling rig, there is no need for pressurized air, such as during the adding or removal of drill rods, relocating the drill rig, setting up the drill rig, operator meal breaks etc. Although there is no need during those periods to circulate compressed air, it is still necessary to drive the power unit in order to power the hydraulic functions.

[0008] In a typical air compressing system used in drilling rigs, the drive connection between the screw compressor and the power unit is such that the screw compressor is driven whenever the power unit is driven, despite the fact that continuous operation of the screw compressor is not necessary when a drilling sequence is not taking place. Equipment which would enable the compressor to be driven at low rpm in such cases, e.g., clutches, and variable speed gear drives, would add considerable cost to the system and have traditionally not been used. However, in an effort to reduce the amount of wasted energy in a relatively low-cost manner, it has been conventional to provide a compressor control system which functions to close the air inlet of the screw compressor to reduce the power required to operate the compressor (e.g., by about 25 percent). In this state, the compressor is said to be “unloaded.” Such compressor control systems used on drill rigs are pneumatically powered, i.e., the components of the compressor control system are operated by compressed air from the compressor itself.

[0009] Conventional air-operated (pneumatic) compressor control systems are unreliable when used in highly humid environments because the moisture in the air drawn into the compressor and supplied to the compressor control system, may condense and corrode components used in the control system. Also, if the temperatures become cold enough to freeze the condensed water, the control system could malfunction. Therefore, it would be desirable to avoid the presence of condensed water in the compressor control system, as well as to provide a more reliable and less expensive compressor control system.

[0010] Conventional pneumatic control systems also create a problem at the beginning of a drilling sequence in conditions where the drilling is to be performed without the use of water to suppress dust. In that regard, it is conventional to slide a dust hood or curtain 28 downwardly along the drill string and onto the ground surface to form a space around the drill string to collect dust, that would otherwise foul the ambient environment, as shown in FIG. 1. The dust hood is an inverted tube, open at its bottom and closed at its top and through which the drill string passes. Cuttings which exit upwardly from the annulus enter the dust hood and are sucked away to a dust separator. It is important that the bottom rim of the hood form a seal with the ground surface to minimize the loss of suction pressure.

[0011] At the beginning of a hole-drilling sequence, the ground surface may be covered with an appreciable quantity of dust, gravel etc. Conventional air-operated compressor control systems are capable of placing the air intake control valve at only two positions, i.e., fully open or fully closed. That means that at the beginning of a hole-drilling sequence, the initial air flow is delivered to the drill bit from the air reservoir, normally loaded to 100 psi pressure. This tends to blow the gravel and dust about at such high speed that a large crater is formed in the ground surface which may extend beneath a flexible bottom rim of the dust hood and thus prevent that rim from sealing properly against the ground. Consequently, the suction pressure within the dust hood is weakened, reducing the efficiency of dust collection.

[0012] Moreover, the abrasive action of the high-speed gravel and dust can accelerate the wearing of the dust hood, the drill bit and the drill string.

[0013] Therefore, it would be desirable to minimize the abrasive action of the dust and gravel at the beginning of a hole-drilling sequence.

SUMMARY OF THE INVENTION

[0014] The present invention relates to a power system which comprises a power unit, a screw compressor, a hydraulic pump, and a hydraulic control system. The screw compressor is connected to the power unit to be driven thereby for compressing air. The screw compressor has an air intake opening and an adjustable air intake valve mechanism for opening and closing the air intake opening. The hydraulic pump serves to pressurize hydraulic fluid. The hydraulic control system is operably connected to the hydraulic pump and to the compressor air intake valve mechanism for utilizing the pressurized hydraulic fluid to adjust the air intake valve mechanism.

[0015] Preferably, the hydraulic control system is operable to maintain the air intake valve mechanism in a selected position, i.e., one of numerous open states in which the air intake opening is open by various degrees, limiting the air flow into the compressor and thus limiting the air delivery.

[0016] Preferably, a source of lubrication oil is provided which communicates with the screw compressor. An oil injection valve controls the amount of lubrication oil admitted into the screw compressor. The hydraulic control mechanism is connected to the oil injection valve for opening the oil injection valve by selected amounts proportional to the degree that the air intake opening has been opened by the air intake valve mechanism.

[0017] An air reservoir is preferably connected to an air outlet of the screw compressor for receiving and storing compressed air received from the screw compressor. The air reservoir includes an oil accumulation section for accumulating lubrication oil entrained in compressed air received from the screw compressor. The oil accumulation section is connected to the screw compressor for recycling lubrication oil to the compressor. The air reservoir includes an air outlet for discharging compressed air, and a minimum pressure valve for maintaining a selected minimum air pressure in the air reservoir. The minimum pressure valve includes a valve element arranged to be pressurized by hydraulic fluid from the hydraulic control mechanism. The valve element is arranged to close the air outlet of the reservoir in response to the air intake valve mechanism being positioned in a state closing the air intake opening, and to open the air outlet in response to the air intake valve mechanism being position in a state opening the air intake opening.

[0018] The above-described control system is of particular benefit when used in a mobile drilling rig, because the minimum pressure valve functions as an air on/off valve on the drill rig.

[0019] The invention also pertains to a method of initiating the drilling of a hole in the ground wherein a drill bit is rotated by a hydraulically driven mechanism to cut through the ground. A flow of compressed air from a screw compressor to the drill bit is established for removing cuttings. A dust hood is positioned against the ground surface in surrounding relationship to the drill bit to capture dust. The improvement involves the actuation of a hydraulic control mechanism to control an air intake valve mechanism of the screw compressor to accurately regulate the compressed air flow pressure such that the flow of compressed air is supplied at a first pressure and volume for minimizing cavitation of the ground surface during the cutting of an initial portion of the hole at the ground surface, and thereafter is supplied at a greater volume and pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The objects and advantages of the invention will become apparent from the following detailed description of a preferred embodiment thereof in connection with the accompanying drawings in which like numerals designate like elements and in which:

[0021]FIG. 1 is a side elevational view of a conventional mobile drilling rig in which the present invention can be utilized.

[0022]FIG. 2 is a schematic view, partially in cross-section, depicting a prior art arrangement of a screw compressor, a power unit, and hydraulic pumps.

[0023]FIG. 3 is a schematic view, partially in cross-section, of the upper end of an air reservoir according to the present invention.

[0024]FIG. 4 is a sectional view taken through an oil injection valve according to the present invention.

[0025]FIG. 5 is a schematic circuit diagram of a power system and hydraulic control system according to the present invention when the compressor is in a loaded state (intake valve open).

[0026]FIG. 6 is a view similar to FIG. 5 when the screw compressor is in an unloaded state (intake valve closed).

[0027]FIG. 7 is a perspective view of a compressor/pump unit in which the present invention can be utilized.

[0028]FIG. 8 is a perspective view of a unit attached to the compressor housing.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

[0029] Depicted schematically in FIG. 5 is a power system for supplying pressurized hydraulic fluid and compressed air, e.g., for use on the drill rig 10. The power system includes a conventional screw-type compressor 32, similar to the above-described compressor 22, which is driven by a power unit, such as the diesel engine 26 shown in FIG. 1. The compressor includes an air intake port 34, the degree of openness of which is controlled by a valve element 35 of an air intake valve mechanism 36 (FIG. 5). An air outlet port 38 of the compressor is controlled by a one-way valve 40 which opens under the urging of compressed air to enable the compressed air to be conducted through a conduit 42 to an air reservoir 44 which is similar to the earlier described reservoir 19. Compressed air is stored in the reservoir 44 at a predetermined pressure controlled by a minimum pressure valve 46 to be discussed in connection with FIG. 3. An outlet 45 for compressed air is disposed at an upper end of the reservoir 44.

[0030] Lubricating oil entrained in the compressed air for lubricating the compressor parts (e.g., drive gears, bearings, etc.) gravitates to an oil accumulation space at the bottom of the reservoir for being recycled back to the compressor through a conduit 48 that contains a compressor oil injection valve 50 to be discussed in connection with FIG. 4. The oil is forced to the compressor by compressed air within the air reservoir.

[0031] The power system also includes hydraulic pumps 52, 54, similar to the previously described pumps 27, that are driven by the engine 26 simultaneously with the screw compressor 32. Those pumps function to pressurize hydraulic fluid from a tank 53 to operate various mechanisms, e.g., the cooling fan 29 via conduit 58, and other hydraulic control valves 60 via conduit 62.

[0032] The power system thus far described is conventional and has traditionally been controlled by a pneumatic control system utilizing available compressed air from the screw compressor 32, as described earlier.

[0033] Depicted in FIG. 5, however, is a preferred embodiment of a hydraulic compressor control system 70 according to the present invention for replacing the standard pneumatic control system. That system 70 utilizes available pressurized hydraulic fluid from the hydraulic pumps 52, 54 by way of fluid take-off conduits 72, 74, respectively. Disposed in the take-off conduits 72, 74 are pressure reducing/relieving valves 76, 78, respectively, which are of conventional design. For reasons to be explained, the valve 76 regulates hydraulic pressure to a higher value P1 than a value P2 regulated by the valve 78 (e.g., 350 psi versus 50 psi). The take-off conduit 72 is connected to the inlet port 80 of a housing 82 which can be mounted on the compressor itself. The inlet port 80 is connected to the inlet side of an electrically-actuated air-on valve 84 that is selectively operable by the drill rig operator between open and closed positions. An outlet of the air-on valve 84 is connected to the hydraulic fluid tank 53 by way of a conduit 86 in which an electrically-driven operator-actuated air-off valve 88 is disposed for selectively opening and closing the conduit 86.

[0034] The outlet of the air-on valve 84 is also connected to the tank 53 through a conduit 90 which contains a limiting valve 92.

[0035] The outlet of the air-on valve 84 is further connected via conduit 94 to one side of a cylinder piston 93 of an air intake valve 96 of the air intake valve mechanism 36. That piston controls the position of the air intake valve element 35 and thus controls the degree of openness of the air intake opening 34. The piston 93 is biased by a spring 95 in a direction for closing the intake element 35.

[0036] In addition, the outlet of the air-on valve 84 communicates, via conduit 98, to the oil injection valve 50, the conduit 98 containing a shuttle valve 99.

[0037] The take-off conduit 74 communicates, via conduit 100, with a side of the piston 93 of the air intake valve mechanism 36, and also with the minimum pressure valve 46 of the air reservoir 44. The take-off conduit 74 also communicates with the oil injection valve 50 via conduit 98 and the shuttle valve 99.

[0038] The minimum pressure valve 46, shown in FIG. 3, comprises a valve piston element 110 biased by a spring 112 to a position engaging a seat 114 to close the air outlet 45. The conduit 100 communicates with one side of the valve element 110 to direct thereagainst the lower hydraulic pressure P2 from the pressure reducing/relieving valve 78 for urging the valve element 110 toward the seat 114. The opposite side of the valve element 110 is exposed to pressure inside of the reservoir 44. When air pressure inside the reservoir exceeds the combined pressures P2 (e.g., 50 psi) of the hydraulic fluid and the spring (e.g., 5 psi), the valve element 100 will raise off the seat 114 to discharge the compressed air through the air outlet 45.

[0039] The oil injection valve 50, shown in FIG. 4, includes a casing 120 forming an oil inlet port 122, an oil outlet port 124, and a closure element 125 biased toward a seat 128 by a spring 130 to block communication between the ports 122 and 124. The oil inlet port is connected to the conduit 48, whereas the oil outlet port 124 communicates with the compressor 32. The valve element 126 is fixed to a rod 132 to which a piston 134 is also affixed. The conduit 98 terminates at the casing 120 to direct hydraulic fluid against the piston 134 in a direction urging the valve element 126 away from the seat 128.

[0040] IN OPERATION, when the engine 26 of the drill rig is started, the compressor 32 and the hydraulic pumps 52, 54 are driven (the compressor 32 being driven regardless of whether or not compressed air is needed, as is conventional). Pressurized hydraulic fluid from the pumps 52, 54 is supplied to desired mechanisms, e.g., the cooling fan 29, and eventually to the other hydraulic devices 60. It will first be assumed that compressed air is not needed, i.e., that a drilling operation is not immediately performed. In that event, the air-on valve 84 is closed by the operator, as shown in FIG. 6, so the only pressure acting on the shuttle valve 99 is from the conduit 74 in which the pressure regulating/relieving valve 78 is disposed which regulates pressure to a lower value P2 (e.g., 50 psi) than the pressure P1 (e.g., 350 psi) regulated by the other pressure regulating/relieving valve 76. Thus, the shuttle valve 99 is shifted by fluid pressure P2 to a position communicating fluid pressure P2 with the conduit 98 and thus with the piston 134 of the oil injection valve 50 to partially open that valve 50 against the pressure of the spring 130. As a result, a limited quantity of lubricating oil is admitted from the reservoir 44 to the compressor 32 (i.e., to flow from the port 122 to the port 124).

[0041] At the same time, fluid pressure P2 is communicated via conduit 100 to a first side of the piston 93 of the valve controller 96 of the compressor intake valve 36 and also to one side of the valve element 110 of the minimum pressure regulating valve 46 of the air reservoir. At this point, the air-off valve 88 will have been operator-shifted to an open position, so the second side of the piston 93 will communicate with the tank 53 (i.e., with atmospheric pressure) via the conduits 94 and 86. As a result, the piston 93 will be maintained by the combined pressures P2 of the hydraulic fluid and the spring 95 in a position closing the valve element 35.

[0042] In this state, the compressor intake opening 34 is closed, so the compressor is unloaded and does not produce an appreciable quantity of compressed air. Also, the minimum pressure valve 46 is also pressurized by fluid pressure P2. That fluid pressure P2, plus the pressure of the spring 112, produces a total pressure (e.g., 55 psi) strong enough to close the outlet 45 of the reservoir 44. A slight (but sufficient) amount of lubrication oil is admitted to the compressor through the partially open oil injection valve 50 to lubricate the rotating compressor screws.

[0043] In the event that a hole-drilling sequence is to be initiated, a flow of compressed air to the drill string is initiated. Initially, only a low-volume and pressure air flow is delivered to the drill string to avoid a severe cratering of the ground surface, as described earlier herein (and which may prevent the dust hood 28 from sealing properly against the ground). That low-pressure air flow is achieved by the operator who momentarily opens the air-on valve 84 (see FIG. 8) a few times by repeatedly nudging the valve 84 to an open state. Each time that the valve 84 is nudged open, the stronger fluid pressure P1 from conduit 72 is conducted through the conduits 94 and 98 (the fluid pressure P1 overcomes the weaker fluid pressure P2 to shift the shuttle valve 99). Accordingly, the pressures acting against the piston 93 of the air intake valve mechanism 36 and the piston 134 of the oil injection valve 50 are gradually increased to open the valves 35 and 50 to a greater (but not maximum) extent. That causes air pressure to build-up in the reservoir 44 which eventually opens the minimum pressure valve 46 and is conducted to the drill string. That air pressure is sufficient to gently blow-out the gravel and dust that has accumulated at the bottom of the hole being drilled and avoids any appreciable cratering of the upper end of the annulus. Consequently, the dust hood 28 is able to properly seal against the ground. Also, the undesirable abrasive sand-basting of the drill bit and drill string is reduced.

[0044] At the same time, the amount of lubrication fluid supplied to the compressor via oil injection valve 50 is proportionally increased.

[0045] Eventually, the operator fully opens the air-on valve 84 to fully open the compressor air intake opening 34 and the oil-injection valve 50 to achieve full-pressure air flow and maximum lubrication oil flow during a drilling sequence.

[0046] The normally-closed limiting valve 92 functions to prevent the pressure in the air reservoir 44 from exceeding a predetermined maximum value, e.g., 90 psi, during a drilling sequence. (Although the minimum pressure valve 46 will open when the reservoir pressure reaches a lower value, the air outlets in the drill bit may become blocked, producing increased pressure in the reservoir.) The reservoir pressure is directed to the limiting valve 92 through a conduit 140. If the air pressure in the reservoir were to exceed the predetermined maximum value, the limiting valve would be shifted to an open state, causing the conduit 94 to be connected to the hydraulic tank 53. As a result, the spring 95 of the air intake control mechanism 36 would be able to displace the piston 93 of the air intake control mechanism 36 to close the air inlet 34 and thereby relieve the pressure in the reservoir 44. Consequently, the limiting valve 92 would reclose, causing the air intake opening 34 of the compressor to reopen. The above-described pressure relief sequence would repeat until the reason for the excessive pressure build-up ceases to occur.

[0047] It may also be desirable, when using the system in cold conditions, to provide a small orifice 148 in the minimum pressure valve 46 which is connected to the hydraulic circuit, e.g., to conduit 72, as shown in FIG. 6. This allows a small amount of hydraulic fluid to circulate and stay at a desired temperature, and thus at a constant viscosity, to prevent the control pressure settings from being adversely affected by viscosity variances.

[0048] The present invention achieves a number of important advantages. By utilizing a hydraulically-operated control system to control the compressor, there is no need to worry about the presence of water condensate in the control fluid. That is, there is no need to worry about the corrosive effect of such condensate, or the possibility that it could freeze and cause malfunctions.

[0049] Furthermore, when the compressor air inlet is closed, the conduit for recycling lubrication oil to the compressor is proportionally closed, thereby reducing the power necessary from the power unit to run the compressor.

[0050] In addition, the minimum pressure valve 46 functions as an on-off valve for the air reservoir 44, thus eliminating the need (and cost) of a separate on-off valve.

[0051] The hydraulically-operated compressor control system enables the compressor air inlet to be set at selected degrees of openness, thereby enabling the air flow at an initial hole-drilling operation to be kept low enough to avoid severe cratering of the ground surface. Thus, the dust hood will seal properly against the ground surface.

[0052] In addition, the hydraulic valves used in the hydraulically-operated compressor control system can be pre-set by the manufacturer. They do not need to be adjusted during the lifetime of the drilling rig, as do pneumatic valves.

[0053] During non-drilling periods, the pressure in the air reservoir is maintained at a lower pressure (e.g., 50 psi) than the pressure (e.g., 100 psi) occurring in air-operated compressor control systems. Therefore, overall energy requirements are reduced.

[0054] The invention can be used in any power system which employs a screw compressor, but is especially useful in power systems which also provide a ready source of pressurized hydraulic fluid. The maximum benefits are achieved when using the invention in a power system that is subjected to humid and/or freezing conditions, such as in earth working environments like mining and drilling. Depicted in FIG. 7 is a compressor/pump unit 150 in which the invention could be used. That unit is basically disclosed in U.S. application Ser. No. 10/259,308, filed Sep. 30, 2002 by the present inventor (the disclosure of which being incorporated by reference herein) and includes a screw compressor 152 and two hydraulic pumps 154 and 155 (only one being visible) that are connected to a common transmission gear mechanism 156. An opening 158 is provided to receive the output shaft of an engine or motor. Attached to a casing of the unit 150 are the elements of the previously described control system. Visible in FIG. 7 are the compressor air intake 34, the air inlet valve 35, the air inlet valve controller 96, the oil injection valve 50, the housing 82, the pressure reducing/relieving valve 78, the air-on valve 84, the air-off valve 88, and the limiting valve 92. It will thus be appreciated that a single, compact unit can be provided which comprises the air compressor, the hydraulic pump(s), the drive transmission for the compressor and pump(s), and the hydraulic control system for the compressor.

[0055] Although the present invention has been described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, deletions, modifications, and substitutions not specifically described may be made without departing from the spirit and scope of the invention as defined in the appended claims. 

What is claimed is:
 1. A power system comprising: a power unit; a screw compressor connected to the power unit to be driven thereby for compressing air, the screw compressor having an air intake opening and an adjustable air intake valve mechanism for opening and closing the air intake opening; a hydraulic pump for pressurizing hydraulic fluid; and a hydraulic control system operably connected to the pump and to the air intake valve mechanism for utilizing the pressurized hydraulic fluid to adjust the air intake valve mechanism of the screw compressor.
 2. The power system according to claim 1 wherein the hydraulic pump is connected to the power unit to be driven thereby simultaneously with the screw compressor.
 3. The power system according to claim 1 wherein the hydraulic control system is operable to maintain the air intake valve mechanism in a selected one of numerous open states in which the air intake opening is open by various degrees.
 4. The power system according to claim 3 further including a source of lubrication oil communicating with the screw compressor, and an oil injection valve for controlling the amount of lubrication oil admitted into the screw compressor; the hydraulic control mechanism connected to the oil injection valve for opening the oil injection valve by selected amounts proportional to the degree that the air intake opening has been opened by the air intake valve mechanism.
 5. The power system according to claim 1 wherein the air intake valve mechanism includes a closure element movably mounted at the air intake opening, and a valve actuator connected to the closure element; the hydraulic control system connected to the valve actuator for utilizing the pressurized hydraulic fluid to adjust the air intake valve.
 6. The power system according to claim 1, further comprising an air reservoir connected to an air outlet of the screw compressor for receiving and storing compressed air received from the screw compressor; the air reservoir including an oil accumulation section for accumulating lubrication oil entrained in compressed air received from the screw compressor; the oil accumulation section connected to the screw compressor for delivering lubrication oil to the compressor; the air reservoir including an air outlet for discharging compressed air, and a minimum pressure valve for maintaining a selected minimum air pressure in the air reservoir; the minimum pressure valve including a valve element arranged to be pressurized by hydraulic fluid from the hydraulic control mechanism; the valve element arranged to close the air outlet of the reservoir in response to the air intake valve mechanism being positioned in a state closing the air intake opening, and to open the air outlet in response to the air intake valve mechanism being positioned in a state opening the air intake opening.
 7. The power system according to claim 1 wherein the hydraulic control system is arranged to receive a first supply of hydraulic fluid at a first pressure, and a second supply of hydraulic fluid at a second pressure; the first pressure being greater than the second pressure; the hydraulic control system arranged to control the intake valve mechanism with the first supply of hydraulic fluid for opening the air intake opening, and to control the intake valve mechanism with the second supply of hydraulic fluid for closing the air intake opening.
 8. The power system according to claim 4 wherein the source of lubrication oil comprises an air reservoir to which oil-containing air is supplied from the compressor; the reservoir including an oil accumulation section to which the oil gravitates; the oil accumulation section connected to the compressor for recycling oil back to the compressor; the air reservoir including an air outlet for discharging compressed air, and a minimum pressure valve for maintaining a selected minimum air pressure in the reservoir; the minimum pressure valve mechanism including a valve element arranged to be pressurized by hydraulic fluid from the hydraulic control mechanism; the valve element arranged to close the air outlet of the reservoir in response to the air intake valve mechanism being positioned in a state closing the air intake opening, and to open the air outlet in response to the air intake valve mechanism being positioned in a state opening air intake opening.
 9. The power system according to claim 8 wherein the hydraulic pump comprises a first hydraulic pump, and further comprising a second hydraulic pump; the hydraulic control system arranged to receive a first supply of hydraulic fluid at a first pressure from the first hydraulic pump, and a second supply of hydraulic fluid at a second pressure from the second hydraulic pump; the first pressure being greater than the second pressure; the hydraulic control system arranged to control the intake valve mechanism with the first supply of hydraulic fluid for opening the air intake opening, and to control the intake valve mechanism with the second supply of hydraulic fluid for closing the air intake opening.
 10. The power system according to claim 9 wherein the hydraulic pumps are connected to the power unit to be driven thereby simultaneously with the screw compressor.
 11. The power system according to claim 3 further comprising an air reservoir connected to an air outlet of the screw compressor for receiving and storing compressed air received from the screw compressor; the air reservoir including an oil accumulation section for accumulating lubrication oil entrained in compressed air received from the screw compressor; the oil accumulation section connected to the screw compressor for delivering lubrication oil to the compressor; the air reservoir including an air outlet for discharging compressed air, and a minimum pressure valve for maintaining a selected minimum air pressure in the air reservoir; the minimum pressure valve mechanism including a valve element arranged to be pressurized by hydraulic fluid from the hydraulic control mechanism; the valve element arranged to close the air outlet of the reservoir in response to the air intake valve mechanism being positioned in a state closing the air intake opening, and to open the air outlet in response to the air intake valve mechanism being positioned in a state opening the air intake opening.
 12. The power system according to claim 11 wherein the hydraulic pump comprises a first hydraulic pump, and further comprising a second hydraulic pump; the hydraulic control system arranged to receive a first supply of hydraulic fluid at a first pressure from the first pump, and a second supply of hydraulic fluid at a second pressure from the second hydraulic pump; the first pressure being greater than the second pressure; the hydraulic control system arranged to control the intake valve mechanism with the first supply of hydraulic fluid for opening the air intake opening, and to control the intake valve mechanism with the second supply of hydraulic fluid for closing the air intake opening.
 13. The power system according to claim 12 wherein the hydraulic pumps are connected to the power unit to be driven thereby simultaneously with the screw compressor.
 14. The power system according to claim 4 wherein the hydraulic pump comprises a first hydraulic pump, and further comprising a second hydraulic pump; the hydraulic control system arranged to receive a first supply of hydraulic fluid at a first pressure from the first pump, and a second supply of hydraulic fluid at a second pressure from the second hydraulic pump; the first pressure being greater than the second pressure; the hydraulic control system arranged to control the intake valve mechanism with the first supply of hydraulic fluid for opening the air intake opening, and to control the intake valve mechanism with the second supply of hydraulic fluid for closing the air intake opening.
 15. The power system according to claim 3 wherein the hydraulic pump comprises a first hydraulic pump, and further comprising a second hydraulic pump; the hydraulic control system arranged to receive a first supply of hydraulic fluid at a first pressure from the first pump, and a second supply of hydraulic fluid at a second pressure from the second hydraulic pump; the first pressure being greater than the second pressure; the hydraulic control system arranged to control the intake valve mechanism with the first supply of hydraulic fluid for opening the air intake opening, and to control the intake valve mechanism with the second supply of hydraulic fluid for closing the air intake opening.
 16. A mobile drilling rig comprising: a mobile platform; a drill string; a mast on the platform for supporting the drill string; a hydraulic motor for rotating the drill string; a hydraulic pump for supplying pressurized hydraulic fluid to power the hydraulic motor; a screw compressor for supplying compressed air to the drill string, the screw compressor having an air intake opening and an adjustable air intake valve mechanism for opening and closing the air intake opening; and a hydraulic control system operably connected to the pump and to the air intake valve mechanism for utilizing pressurized hydraulic fluid to adjust the air intake valve mechanism.
 17. The mobile drilling rig according to claim 16 wherein the hydraulic pump is connected to the power unit to be driven thereby simultaneously with the screw compressor.
 18. The mobile drilling rig according to claim 16 wherein the hydraulic control system is operable to maintain the air intake valve mechanism in a selected one of numerous open states in which the air intake opening is open by various degrees.
 19. The mobile drilling rig according to claim 18 further including a source of lubrication oil communicating with the screw compressor, and an oil injection valve for controlling the amount of lubrication oil admitted into the screw compressor; the hydraulic control mechanism connected to the oil injection valve for opening the oil injection valve by selected amounts proportional to the degree that the air intake opening has been opened by the air intake valve mechanism.
 20. The mobile drilling rig according to claim 16 wherein the air intake valve mechanism includes a closure element movably mounted at the air intake opening, and a valve actuator connected to the closure element; the hydraulic control system connected to the valve actuator for utilizing the pressurized hydraulic fluid to adjust the air intake valve.
 21. The mobile drilling rig according to claim 16, further comprising an air reservoir connected to an air outlet of the screw compressor for receiving and storing compressed air received from the screw compressor; the air reservoir including an oil accumulation section for accumulating lubrication oil entrained in compressed air received from the screw compressor; the oil accumulation section connected to the screw compressor for delivering lubrication oil to the compressor; the air reservoir including an air outlet for discharging compressed air, and a minimum pressure valve for maintaining a selected minimum air pressure in the air reservoir; the minimum pressure valve including a valve element arranged to be pressurized by hydraulic fluid from the hydraulic control mechanism; the valve element arranged to close the air outlet of the reservoir in response to the air intake valve mechanism being positioned in a state closing the air intake opening, and to open the air outlet in response to the air intake valve mechanism being positioned in a state opening the air intake opening.
 22. The mobile drilling rig according to claim 16 wherein the hydraulic pump comprises a first hydraulic pump, and further comprising a second hydraulic pump; the hydraulic control system arranged to receive a first supply of hydraulic fluid at a first pressure from the first pump, and a second supply of hydraulic fluid at a second pressure from the second hydraulic pump; the first pressure being greater than the second pressure; the hydraulic control system arranged to control the intake valve mechanism with the first supply of hydraulic fluid for opening the air intake opening, and to control the intake valve mechanism with the second supply of hydraulic fluid for closing the air intake opening.
 23. In a method of initiating the drilling of a hole in the ground wherein a drill bit is rotated by a hydraulically driven mechanism to cut through the ground, a flow of compressed air from a screw compressor to the drill bit is established for removing cuttings, and a dust hood is positioned against a ground surface in surrounding relationship to the drill bit to capture dust; the improvement wherein a hydraulic control mechanism is actuated to control an air intake valve mechanism of the screw compressor to regulate the compressed air flow and pressure such that the flow of compressed air is supplied at a first volume for minimizing cavitation of the ground surface during the cutting of an initial portion of the hole at the ground surface, and thereafter is supplied at a greater volume. 